This invention relates to a process for consolidating fiber-reinforced thermoplastic composite panels.
The production of thermoplastic composite panels containing fiber reinforcement and thermoplastic resin requires the application of heat sufficient to soften the thermoplastic resin together with the application of sufficient pressure, usually in a mold press, to cause the softened resin to wet out and encapsulate the reinforcing fibers. Many thermoplastic resins are highly viscous in the melt at a temperature near their softening point and must be subjected to high pressures to flow to the corners of the mold and adequately wet out the fiber reinforcement. Heating the resin to higher temperatures to improve the flowability trades off against the curing of the resin and/or exceeding the metallurgical limits of commonly used mold materials.
Another problem caused by the viscous thermoplastic resin resides in the difficulty of obtaining uniform pressure distribution by the mold press for uniform consolidation of the panel. For large panels, for example, panels measuring more than about 10 inches on each side, imperfect alignment of the mold press platens causes non-uniform wetout and consolidation of fibers and resin in the panel. The lack of uniform consolidation is undesirable for applications which require high quality parts, such as in the aerospace industry. The degree to which uniform consolidation has been effected during the molded cycle can be determined quantitatively by the C-scan technique, which is a nondestructive acoustic transmission test.